Changing Currents in the North Atlantic

A recently published paper suggests that the Atlantic Meridional Overturning Circulation (AMOC) (also called the Gulf Stream System) is slowing down. What is the AMOC? It’s a current that brings warm waters up to the North Atlantic from the Gulf Stream, then sinks and heads south as the water gets colder, saltier, and denser. Like a conveyor belt, it pulls warm water north as it sinks, bringing mild winters to places like the UK and Nordic countries. Because this current spans the globe, however, changes in the North Atlantic have implications throughout the Atlantic Ocean basin and even down to Antarctica.

Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures (Image by R. Curry, CC-BY-3.0).

Why is the AMOC slowing down? The argument is that, with increasing melt from the Greenland Ice Sheet and glaciers in the Arctic archipelago, more fresh water is entering the North Atlantic and reducing the densification of the water, which needs to be salty to densify. The freshwater means that the North Atlantic water isn’t dense enough to sink and pull the warm water up north behind it. In fact, there is a section of the North Atlantic, just south of Greenland, called the Cold Blob, which suggests that less warm water is making its way north.

NASA imagery of the Cold Blob in 2015 (public domain image).

This slowing and even collapse of the AMOC has happened in the past, during a period called the Younger Dryas. This was the last major cold episode on Earth and is thought to have been triggered by a meltwater flood into the North Atlantic. As the continental glaciers were retreating, they formed large meltwater lakes at their margins. One theory is that one of these lakes, Lake Agassiz, drained out the St. Lawrence seaway and into the North Atlantic and reduced the AMOC. More recent research, however, has noted that freshwater from the Arctic was likely a stronger contributor to the decline in the AMOC during the Younger Dryas. Either way, it was fresh water from glacier melt that caused the decline in the AMOC, which is the same mechanism used to explain the slowdown we see today.

Flow pathways of ice marginal lakes during the Younger Dryas. The Arctic route was likely the driver of an AMOC shutdown.

Why do we care about it? Because the AMOC affects weather and climate around the world. This great article in the NY Times, complete with fancy visuals, notes that changes in the AMOC can result in:

“stronger hurricanes barreling into the Southeastern United States, and perhaps most ominously, reduced rainfall across the Sahel, a semi-arid swath of land running the width of Africa that is already a geopolitical tinderbox.”

The article briefly mentions the loss of winter heat transferred to northern Europe and the UK, though notes that the loss of warmth could be compensated for by global warming. It also mentions the possibility of more extreme summer heat waves, as the Cold Blob interacts with the jet stream to shift its trajectory to the south, bringing warmer temperatures to Europe. This illustrates the complexity if the climate system, if changes in the AMOC can lead to both heating and cooling in northern Europe.

Not everyone is convinced that the AMOC is slowing down. Some argue that we don’t have a long enough data record to quantify a slowdown. But it’s something to watch closely given its potential global impacts. 

Note this week’s featured image is of the Greenland Ice Sheet and was taken by C. Zenino, CC-BY-2.0.

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